A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The present invention finds application, by way of example only, in a projection optics assembly, which is typically part of a lithographic apparatus. A projection optics assembly that includes a support frame is known. Conventionally, depending on the particular lithographic apparatus, the projection optics assembly includes a reference frame and a plurality of sensor frames that are mounted in the reference frame. The sensor frame is a frame adapted to support various components. Typically, the support frame supports a first element in a spaced relationship with respect to a second element. The first and second elements are positioned and fixed in a very stable manner. Conventionally, in order to position the elements with respect to each other, spacers are needed to provide an interface surface. The spacers are disposed in the support frame. The elements to be disposed in the frame have a predetermined functionality, which determines how they are to be orientated in the support frame. For example, a sensor element for sensing a position in a certain direction must be oriented in a particular way with respect to that direction. Conventionally, spacers are provided to position the element according to its particular functionality. Currently, each position sensor is provided with six spacers. Attaining accurate positioning after mounting of the mirror modules in the projection optics assembly is done through employing additional spacers. The dimensions of each spacer are individually determined in accordance with the desired spacing it performs, as governed by the function of the element. Each spacer is then individually manufactured, typically by grinding the spacer material (which is for example, the material ZERODUR®) to the precise dimensions. Typically, several tens of spacers are required in order to fixedly position the elements with respect to one another on the support frame.
It has been found that in conventional projection optics assemblies, the use of spacers presents certain problems. The greater the number of spacers required, the greater the problems. In particular, a large number of spacers are needed to mount a sensing element that senses the position of an optical element, such as a mirror. One problem is that the use of spacers is expensive. In particular, the number of man hours required to determine the dimensions and to grind the spacers to size, and also to mount the spacers in the support frame is large. Furthermore, the large number of parts adds to the complexity of the resulting projection optics assembly, which may lead to long term stability issues regarding the positioning of the elements with respect to one another.